The present invention relates to a process for manufacturing fiber structure electrode plates for use in positive and negative electrodes of electrochemical current accumulator sources, particularly in open or closed, gastight nickel cadmium accumulators.
In the manufacturing of cells of a prismatic shape, when fiber structure electrodes are used, because of excessive variations in the individual manufacturing steps, among other things, in the thickness of the individual components (principally of the positive and negative electrodes), after the mounting of the plate stack and the installation of the plate stack with the separators and the recombining devices, the cell housing will be so thick that several cells of this type cannot be installed into an existing steel receptacle, such as a battery tray. The excessive tolerances in the manufacturing of electrodes have additional disadvantages during the construction and operation of cells with parts produced in this manner; these include, for example: that the laid-out electrolyte quantity corresponding to the volume of the planned housing is not coordinated with the volume of the actual expanded housing; that the actual porosities and hollow-space distributions in the real cell do not correspond to the theoretical and calculated values; that shifts occur in the amount of the dischargeable capacity and energy for various load conditions; that lower Ah and Wh yields result; that a changed inside pressure of the cell occurs (usually connected with a decreased lifetime of the cell); that, because of the undefined electrode geometry, no uniform electrode gap is ensured; that a non-uniform distribution of the amount and concentration of the electrolyte occurs; that the pressure conditions affecting the installed separators differ and therefore a uniform electrolyte storage (intake capacity) is disturbed; or that an imbalance builds up of the part of the charge and discharge reserve of the negative electrode, so that the negative electrode is larger than the positive electrode.
Variations in the manufacturing process occur, among other things, due to the unbending of a plate during filling, insufficient removal of the excess paste from the surface of the electrode after the pasting, or an oozing-out of paste before the drying. If such variations are not eliminated prior to assembly, a high failure rate of the cells occurs due to short circuits, which are usually caused at only a few locally occurring points in the plate stack, and often do not break through until after the operation of several cycles of the cell.
In this respect, it is known, for example, from the German Patent Document DE-AS 11 08 759, that a non-metallized foamed-material structure plate with a diverting system which is to be installed separately is to be used in a process for manufacturing electrodes for accumulators, particularly for lead accumulators.
German Patent Document DE-AS 10 63 233 discloses an electrode for alkaline accumulators in which the carrier of the active mass consists of metal filaments which are sintered together with one another or of similar structures, as well as a process and apparatus for manufacturing such carrier. However, as is disclosed in this text, the pressing-together of electrodes of this type, after the activation to increase the conductivity and capacity, is possible only with limitations, or is hardly possible without the risk that the junction points which are welded together with one another tear apart. As a result the cohesion of the sintered structure is endangered.
German Patent Document DE-AS 15 96 240, discloses an electric accumulator in which the carriers of the electrodes consist of a fabric or felt as the fiber material, and the ductile electrodes are compressed in the accumulator case. For this purpose, the carrier is subjected to a compression after depositing of the active mass (see Column 7, Line 30 et seq.). This text contains no information, however, on the process conditions of the suggested compression, or on how many process steps are required for this purpose.
German Patent Document DE-PS 30 05 725 relates to a foam structure electrode with a sponge-type deposit of a nickel metal matrix, for galvanic elements. However, in the case of sponge-type structures (and metallic sponge-type structures, as well), after the pressing-together, there is generally a considerable memory effect with respect to the shape, so that they resume their original shape when the outside pressure is eliminated.
In addition, a Publication of the 16th International Power Sources Symposium 1988, Page 1 et seq., discloses sponge-type nickel electrodes to which the above-mentioned criticism also applies.
It is therefore an object of the invention to provide a process for manufacturing dried fiber structure electrodes, which are filled with an active mass, and for manufacturing their coating, in which the above-mentioned disadvantages, such as excessive variations in thickness, and the resulting non-uniform pressure and distribution conditions (electrolyte) and an unfavorable installation of the premounted plate sets into the cell housing, are eliminated or at least reduced. It is also an object of the invention, to provide a process by which the fiber structure electrode plates can be manufactured efficiently and with precise measurements for high standards. It is also an object of the invention to provide cells of a prismatic shape which do not lose their required characteristics prematurely due among other things to soft short circuits.